Some of the objectives of my invention have been accomplished by providing a telemetering system operating in the "regime of hydraulic shock waves"(FIGS. 2A and 2B) as distinct from the regime of slow variations of pressure (FIGS. 1A and 1B). My invention is based on an observation that by gradually increasing the speed of operation of the valve a certain transition is reached which divides the two regimes. This transition has been numerically defined. In accordance with my invention the hydraulic shock information while drilling is in process. These shock waves are produced by a very rapidly acting (for all practical purposes almost instantaneously acting) bypass valve interposed between the inside of the drill string and the annulus around the drill string. When the bypass valve suddenly opens, the pressure in the immediate vicinity of the valve drops and then returns to normal almost instantaneously and a sharp negative pulse is generated, and conversely, when the bypass valve suddenly closes, a sharp positive pulse is generated. Elasticity of mud column is employed to assist in the generation and transmission of such shock waves. The phenomenon is analogous to the well known water hammer effect previously encountered in hydraulic transmission systems. (see for instance John Parmakian on "Water Hammer Analysis", Prentice Hall, Inc., New York, N.Y. 1955 or V. L. Streeter and E. B. Wylie on "Hydraulic Transients" McGraw-Hill Book Co., New York, N.Y.) Another objective of my invention is to represent downhole information in form of a succession of wavelets each of which comprises two distinguishable pulses of opposite polarity. The "negative" pulse represents the opening of a bypass valve and the "positive" pulse represents the closing of the bypass valve.
Significant features of my invention such as the generation and detection of hydraulic shock waves are shown schematically in FIGS. 2A and 2B. The graph in FIG. 2A shows the openings and closings of a fast acting shock wave producing valve, whereas the graph of FIG. 2B shows pressure variations detected at the earth's surface and resulting from the operation of the valve as in FIG. 2A. Symbols such as A.sub.1, B.sub.1, C.sub.1, D.sub.1, t.sub.a.sup.(v), t.sub.b.sup.(v), t.sub.c.sup.(v), t.sub.d.sup.(v), T.sub.a.sup.(v), T.sub.b.sup.(v) and T.sub.t.sup.(v) in FIG. 2A have a similar meaning as the corresponding symbols in FIG. 1A. However, the time scales in FIGS. 1A, 1B, 2A and 2B have been considerably distorted in order to facilitate description, and in the interest of clarity of explanation.
The first thing which should be noted in examining FIG. 2A is that the times of opening and closing of the valve in accordance with my invention are by several orders of magnitudes shorter than the corresponding times obtained by means of the motorized valve as reported in connection with FIG. 1A. In the arrangement previously suggested (as in FIG. 1A) one had T.sub.a.sup.(v) =1 second whereas in accordance with my invention as in FIG. 2A one has T.sub.a.sup.(v) =5 milliseconds. A similar situation applies to the time interval during which a valve remains open. In the arrangement previously suggested (as in FIG. 1A) one had T.sub.b.sup.(v) =2 seconds whereas in FIG. 2A one has T.sub.b.sup.(v) =100 milliseconds. Thus, for all practical purposes, the openings and closings of the valve in FIG. 2A may be considered as instantaneous or almost instantaneous.
Rapid or almost instantaneous openings and closings of the valve have an important and far reaching influence on the performance of a telemetering system in a measuring while drilling operation. The pressure variations detected at the earth's surface in accordance with my invention (FIG. 2B) show no similarity whatever to the pressure variations obtained by means of a slow acting valve (FIG. 1B). I have previously pointed out the existence of equations (6), (7), and (8) which show relationships between the events illustrated in FIG. 1A and those illustrated in FIG. 1B. Analagous relationships do not exist between the events in FIGS. 2A and 2B.
As shown in FIGS. 1A and 1B, the opening of the valve produced a corresponding decrease in the mud pressure at the surface of the earth, and conversely, the closing of the valve produced a corresponding increase in pressure.
For the sake of emphasis I wish to repeat that in the prior art the opening of the valve produced a single event namely a decrease in pressure and the subsequent closing of the valve produced another single event--an increase in pressure. On the other hand in my invention the fast opening of the valve as in FIG. 2A produces two events: a rapid decrease and subsequent increase in pressure (negative pulse "M" as in FIG. 2B). This is in contrast to the case shown in FIG. 1A and FIG. 1B where an opening and a subsequent closing of the valve is required in order to produce a decrease and a subsequent increase in pressure. Furthermore, the fast closing of the valve as in FIG. 2A produces an increase and a subsequent decrease of the mud pressure (positive pulse "N" as in FIG. 2B). Such an increase and subsequent decrease in pressure does not occur in the arrangements suggested in the prior art. In my invention, there are two shock waves produced by a single operation of the valve. A wave form such as shown in FIG. 2B, which comprises both a negative and a positive pulse, will be referred to in this specification as a "valve wavelet". Pressure pulses associated with a valve wavelet have an onset rat of several thousand psi/sec. and are of short duration.
It is of interest to point out the rapidity of the phenomena associated with the observed valve wavelets. The times involved in FIG. 2B are as follows:
The time interval T.sub.n.sup.(s) representing the "length" of the negative pulse "M" (or the positive pulse "N") is 100 milliseconds, whereas the time interval T.sub.m.sup.(s) from the appearance of the negative pulse "M" to the appearance of the positive pulse "N" is 110 milliseconds. Thus, the total period of flow as shown in FIG. 2B; i.e., EQU T.sub.u.sup.(s) =T.sub.n.sup.(s) +T.sub.m.sup.(s) ( 9)
is 210 milliseconds whereas the total period of flow as shown in FIG. 1B (see equation 5) was T.sub.t.sup.(s) =4 seconds.
The graphs in FIGS. 1A, 1B, 2A, and 2B have been simplified and idealized by eliminating ripples and other extraneous effects. It should also be noted (see FIG. 2B) that the bypass valve is at least partially open during the time interval from t.sub.1.sup.(s) to t.sub.4.sup.(s). During this time interval, there is a slow pressure decline which is eliminated at the detection point by an appropriate filter. Such a pressure decline is not shown in the graph of FIG. 2B.
It should also be pointed out that the numerical values attached to FIG. 2A and 2B are given merely as an example. These values should not be interpreted as restricting my invention to any particular example given.
The process as explained in connection with FIGS. 2A and 2B has been referred to above to as relating to a "regime of hydraulic shock waves". Thus, a distinction is made between the regime of hydraulic shock waves as in FIG. 2A and 2B and the regime of slow variations of pressure as in FIG. 1A and 1B.
By providing a regime of hydraulic shock waves, I obtained a telemetering system by means of which large amounts of information can be transmitted per unit of time. Such a system is considerably better adapted to satisfy current commercial requirements than the one which is based on the regime of slow variations of pressure.
The valve, in accordance with my invention, is operated by the output of one or more sensors for sensing one or more downhole parameters in the earth's subsurface near the drill bit. One single measurement of each parameter is represented, by a succession of valve wavelets. Each valve wavelet corresponds to a single opening and closing of the valve.
The succession of valve wavelets (which represents the useful signal) when detected at the earth's surface is usually mixed with various interfering signals such as those produced by the operation of the pump and by other drilling operations. In a typical drilling arrangement, a large pump located at the surface is used to pump drilling mud down the drill stem through the bit and back to the surface by way of annulus between the drill pipe and the well bore. The interference effects due to the pump are eliminated in this invention by a process which takes into account the periodicity of these effects. Other effects associated with drilling operations usually appear as noise signal comprising a relatively wide frequency spectrum. This noise signal is in some instances white noise and in other instances it departs considerably from white noise. A digital filtering system which may be a matched filter or a pulse shaping filter or a spiking filter is employed to remove the noise signal. The matched filter maximizes the signal to noise ratio at the reception point, a pulse shaping filter minimizes the mean square difference between a desired output and the actual output, whereas a spiking filter transforms the useful signal by contracting it into one which is sufficiently sharp so that it can be distinguished against a background noise. A special technique is applied to adapting these filters to the objectives of this invention. Such a technique requires storage and subsequent reproduction of two reference signals. The first reference signal is a wavelet produced by the opening and closing of the valve and the second reference signal represents noise due to the drilling operations. Detection and storage of the first reference signal is obtained by removing the weight on the bit and stopping the actual drilling (but maintaining the mud pumps in normal action. Thus, a signal is obtained which is free from the ambient noise. Detection and storage of the second reference signal is obtained when drilling is in progress during a period of time when the valve is closed. An appropriate digital computing system is arranged to receive the data representing one or both of these reference signals, and derives from the data a memory function for the matched filter, for the pulse shaping filter, or for the spiking filter.
The novel features of my invention are set forth with particularity in the appended claims. The invention both as to its organization and manner of operation with further objectives and advantages thereof, may best be presented by way of illustration and examples of embodiments thereof when taken in conjunction with the accompanying drawings.